U.S. Pat. No. 4,236,607 (Halwes et al.) discloses a spring-tuning mass vibration isolator in which force cancellation is accomplished by hydraulically amplifying the inertia of a tuning mass. Other hydraulic inertial vibration isolators are known in the art, such as those disclosed in U.S. Pat. Nos. 4,811,919 (Jones) and 5,174,552 (Hodgson et al.).
The preferred embodiment of the Halwes vibration isolator utilizes mercury both as hydraulic fluid and as the tuning mass. While mercury is quite dense and has low viscosity, both of which are advantageous properties, it is extremely corrosive and toxic. As a result, other, lower density liquids have been used in such vibration isolators. Unfortunately, the use of a lower density liquid requires that the size of the vibration isolator be increased to compensate for the liquid's decreased density.
Halwes also discloses another embodiment of the vibration isolator which utilizes a high density metal slug as the tuning mass and a relatively low density liquid as the hydraulic fluid. The size of a metal tuning slug vibration isolator is comparable with that of a mercury vibration isolator, and the metal tuning slug vibration isolator lacks the disadvantages associated with mercury. However, large amplitude vibration and/or vibration at frequencies near the natural frequency of the vibrating body-vibration isolator-isolated body system (the "system") can cause excessive metal tuning slug motion ("overtravel"), resulting in the metal tuning slug contacting the end sections of the vibration isolator. This can damage the vibration isolator, possibly causing it to fail. A means for limiting overtravel of the metal slug would minimize the possibility of such an occurrence.
A hydraulic inertial vibration isolator provides excellent vibration attenuation at a particular vibration frequency, the isolation frequency. However, vibration attenuation decreases rapidly as the vibration frequency varies from the isolation frequency. Thus, the vibration isolator is effective over a relatively narrow range of vibration frequencies. A means for varying the isolation frequency would allow the vibration isolator to be effective over a broader range of vibration frequencies.
Although hydraulic inertial vibration isolators are generally designed to have minimal damping, some damping is present. As a result, the degree of vibration isolation at the isolation frequency is less than ideal. A means for adding energy to the vibration isolator to compensate for losses due to damping would allow the unit to provide substantially ideal vibration isolation, i.e., 100% isolation.
It is an object of the present invention to provide a vibration isolator which utilizes a metal slug as a tuning mass and in which the amplitude of the metal slug's motion is limited to minimize the possibility of damage to the vibration isolator.
It is a further object of the present invention to provide a vibration isolator which allows the isolation frequency to be varied.
It is a further object of the present invention to provide a vibration isolator in which vibration isolation at the isolation frequency is virtually 100%.